A typical method for processing exposed silver-based photographic material comprises passing the exposed material through a bath of developer liquid and subsequently through a bath of fixer liquid. The fixer usually contains thiosulphate, which removes silver from unexposed parts of the photographic material. Because of the consumption of thiosulphate, it is necessary to periodically regenerate used fixer liquid by the addition of fresh fixer thereto if consistent fixing results are to be achieved. It is desirable that the quality of the treatment liquid in the fixing bath remains at an optimum level during processing, even when the processing apparatus is being used erratically. In other words it is desirable to calculate a desired fixer replenishment rate.
It has been proposed, for example in Japanese patent application JN03053245 (Fuji Shashin Film), to calculate the quantity of fresh fixer which needs to be added on the basis of the nature of the images carried on the photographic material which are to be fixed during the coming period, in particular upon the image density. Information on the image density may be obtained from the imaging apparatus which produces the exposed photographic material, especially where this is a laser exposure device.
However, this method of calculating replenishment rates is not wholly satisfactory. The actual depletion of active components in the fixer during fixing of the photographic material may not correspond exactly to that predicted from the image data. This may occur, for example, where the photographic material is not completely developed, leaving greater amounts of silver halide in emulsion than might have been predicted, and therefore consuming higher than predicted amounts of active components in the fixer. Even if these differences are small, over time their effect may be cumulative, resulting in significant under- or over-replenishment. Over time, the concentration of active components in the fixer may change as a result of oxidation and evaporation, and these changes tend to occur even at times when the fixer is not being used. There is therefore a risk that insufficient fresh fixer is added to maintain the activity of the fixer at an optimum level.
Furthermore, the problem of optimising fixer regeneration is complicated by the carry-over of developer into the fixing bath, which may deactivate some components of the fixer. Depending upon the design of the processing apparatus, particularly the squeegee rollers used therein, the amount of this carry-over may depend upon the shape and quantity of photographic material processed, independent of the image density thereon. The error in calculated replenishment rates will be particularly significant when replenishment rates are low, for example when average image density is high.
Some processing methods use an electrolytic technique to remove the silver which builds up in the fixer as it is used, for example by continuously circulating the fixer through an electrolytic silver recovery cell. In the silver recovery cell, not only is silver extracted at the cathode and thiosulphate is regenerated, but some sulphite is destroyed at the anode. The amount of silver extracted during a given period, which can be calculated from the electrical energy input into the cell, is related to the image density of the photographic material which has been processed during that period. It might be supposed therefore, that the calculation of the fixer replenishment rate can be based upon the quantity of silver extracted by this electrolytic technique.
However, electrolysis results are generated rather slowly, compared with the rate at which the silver is extracted from the photographic material. If a peak in processing throughput should occur, the electrolysis may fall behind, resulting in an inadequate level of replenishment being calculated and therefore starvation of the fixer may occur. Furthermore, should the activity of the fixer fall below optimum, this results in poorer fixing and consequently a lower level of silver being brought into solution to be extracted by electrolysis. The consequential fall-off in silver extraction is not immediately an indication of poor fixer activity.